This invention relates to internal combustion engine cylinder pressure sensors, more particularly a non-intrusive sensor of the "annular insert" type which measures flexure of a first wall defining in part a cylinder relative to a second relatively rigid wall of an internal combustion engine housing component. Such a sensor is disclosed in U.S. Pat. No. 4,969,352 to Sellnau and assigned to the assignee of the present invention. In the disclosure of that patent such a Sensor finds particular utility when mounted in a spark plug access well.
It is known in the field relating to internal combustion engines to utilize cylinder pressure sensors for monitoring and controlling various processes, among them being combustion knock, misfire and associated combustion dilution and combustion phasing. Delivery of fuel and exhaust gas recirculation and timing of ignition may then be optimized accordingly. In order for cylinder pressure sensing to find its way into the mainstream of commercial automotive applications, it must be cost effective. Advances to making pressure sensing cost effective have been made as shown in Sellnau wherein simple modifications to pre-existing engine head features provide necessary mounting provisions for non-intrusive sensors. Another way to advance cost effectiveness is by reducing piece price. This is especially true where multiple sensors (i.e. one per cylinder) are to be used in a single vehicle. Therefore, innovative designs which reduce piece price by simplifying manufacturing and assembly are highly favored.
Sellnau '352 shows a sensor having a castellated lower portion effective to allow axial compressive forces to act on a trapped sensing element while bypassing torsional loads around the load sensing element. Another approach accomplishing the same is also shown wherein an integral axial extension from the major portion of the annular insert extends downward therefrom and is bent in an outward flange arrangement to trap the sensing element between the flange portion and the major portion of the annular insert. Such arrangements accomplish the objective of bypassing torsional loads around the sensing element, however, they also leave the sensing element and electrical connections exposed to infusion of such common deleterious materials as oil, road salt and water. Additionally, such arrangements are relatively complex and costly and may not be necessary where the sensing element can sufficiently cope with the torsional loads expected. It is desirable to simplify sensor packaging and ensure that the sensing element and electrical connections are adequately shielded from electromagnetic interference and sealed against contaminant infusion.
In order to obtain a signal from such a sensor it is necessary to electrically couple to the sensing element. One approach shown in related art cylinder pressure sensors provides for electrical coupling to each of two sides of a piezoelectric sensing element by electrodes or metal traces attached thereto, thereby requiring at least two sensor leads and providing an engine ground-isolated sensor with a differential signal. Reducing the number of lead wires and simplifying electrical coupling to a single-ended configuration would reduce failure modes, increase reliability and may decrease the complexity of overall engine dress wiring. Also, the sensor signal would not be subject to stray capacitive effects precipitated by engine ground noise. For these reasons, it is desirable to simplify electrical coupling including reducing the number of lead wires.
It is possible in such an arrangement that significant noise may be induced upon the high impedance sensor from the high voltage ignition system. This is particularly true where a spark plug access well is the location of choice for such a sensor. It is therefore desirable to reduce noise induced upon the pressure sensor by the high voltage ignition system components proximate thereto.